1. Technical Field
The present invention relates to a technique for fabricating gratings in optoelectronic and optical devices and, more particularly, to a fabrication technique utilizing a combination of electron-beam lithography and holographic optical exposure.
2. Description of the Prior Art
Corrugated waveguide gratings are considered to be key elements for many optical devices, such as filters, distributed feedback (DFB) lasers, and distributed Bragg reflector (DBR) lasers, where such devices are expected to play a significant role in future lightwave communication systems. The fabrication of corrugated waveguides is often accomplished utilizing a holographic technique, as taught in U.S. Pat. No. 4,517,280, entitled "Process for Fabricating Integrated Optics", issued to K. Okamoto et al. on May 14, 1985. As is known in the art, such a holographic technique relies on the exposure of resists using two interfering UV laser beams. The interference pattern allows for the formation of submicron-pitch gratings, but is limited in the different variety of features which can be made during a single process step.
Another technique that has been used extensively to fabricate corrugated grating structures for optoelectronic or optical device applications is direct-write electron-beam (c-beam) lithography. In this technique, the desired periodic features are exposed by an electron beam directly in an e-beam sensitive resist (such as PMMA) that covers the substrate which is to contain the final grating structure. Indeed, this technique has demonstrated the desired versatility of forming abrupt phase shift, as well as multiple grating pitches on a single wafer. See, for example, an article entitled "Performance of 1.5 .mu.m .lambda./4-Shifted DFB-SIPBH Laser Diodes" by C. E. Zah et al., appearing in Elector Letters, Vol. 25, No. 10, May 1989, at pp. 650-1. The principal drawback of this technique is the requirement that each wafer or substrate be individually processed. That is, each must experience the entire direct-write electron-beam process, including loading the sample into a vacuum chamber, as well as the precision alignment and (usually) very lengthy direct-write exposure process itself. As such, the direct-write electron-beam exposure of large-area features, such as gratings for optoelectronic or optical devices, is not viewed as a manufacturable process.
An alternative technique is discussed in an article entitled "Novel method to fabricate corrugation for a .lambda./4-shifted distributed feedback laser using a grating photomask", by M. Okai et al. appearing in Applied Physics Letters, Vol. 55, No. 5, July 1989, at pp. 415-6. In this case, a precision ruling machine is used to form a metal substrate with triangular grooves. This triangular groove pattern is then transferred to a transparent polymer film, which results in a triangular-wave phase mask. When illuminated off-axis, this mask will generate a diffracted beam which will interfere with the transmitted beam to produce an interference pattern similar to that observed in the conventional two-beam holographic interference method. The modulation resulting from the interference pattern in the intensity on the far side of the mask is intended to expose resist which has been placed on the actual sample to be patterned. However, the triangular phase mask is incapable of generating the equal intensities of transmitted and diffracted beams which are required to obtain a high contrast-ratio interference pattern suitable for resist exposure. This is remedied, at the expense of overall intensity and thus longer exposure time, by evaporating a metal film, off-axis, to asymmetrically coat the triangular grating features on the polymer mask, and thus equalize the transmitted and diffracted beam intensities. This technique is also currently understood to require a laser exposure system similar to the conventional two-beam holographic interference method. The utilization of the ruling machine to form the gratings allows for the mask to be modified, as desired, to incorporate pitch changes (and therefore phase shifts) within the grating during a single print.
Although the Okai et al. mask provides for improved fabrication techniques, the flexibility of their approach is still somewhat limited, especially by the mechanical ability of the ruling machine with respect to the number of grooves per millimeter which may be formed.